It is a common practice in some industries to ship or transport certain liquids in a shipping container or tank which is also used as a supply tank for the same liquid at a work station in the user's facility. For example, in the auto industry paint is shipped by truck or rail from the paint manufacturer's facility to the auto manufacturer's facility where it is used for painting cars. The tanks may hold about two hundred gallons or more and are typically of rectangular shape for efficiency in shipping. When the tanks reach the destination facility, they are placed near a work station and used as the supply tank for the painting operation on the production line. The paint in the tank must be stirred or agitated at the work station in order to maintain a uniform composition throughout the tank. For this purpose, the tank is provided with a mixing device which comprises an impeller inside the tank mounted on a shaft which extends through a bearing in the top of the tank for connection with an externally mounted drive motor. The drive motor is part of a drive module which is adapted for connection to the impeller shaft when the tank is installed at the work station and is disconnected when the empty tank is removed from the work station. Such mixing tanks meet the needs of industry for various liquids which require mixing or agitation at the work station where the liquid is used. However, there has been a long standing problem of providing a trouble free seal for preventing leakage of the liquid around the agitator shaft during shipping and also during operation of the mixing device.
A prior art arrangement for sealing the agitator shaft in a mixing tank is disclosed in the Bissell U.S. Pat. No. 2,137,328 granted Nov. 22, 1938. In this arrangement, a cylindrical stuffing box is mounted in the wall of the tank and the agitator shaft extends through the stuffing box. The shaft seal comprises a plurality of packing rings which are stacked on one another in the sleeve of the stuffing box. The rings are compressed against the bottom of the sleeve by a clamping ring which is tightened by a threaded fastener This type of arrangement is not satisfactory because the packing rings in the stuffing box require a great deal of cleaning and maintenance to ensure that a good seal is achieved during transport and during operation of the agitator. A similar shaft seal arrangement is disclosed in the Werner U.S. Pat. No. 4,127,310 granted Nov. 28, 1978.
Another prior art arrangement for an agitator shaft seal is described in the Boutros et al patent U.S. Pat. No. 2,911,240 granted Nov. 3, 1959. This patent refers to the prior art stuffing box construction using packing rings and a gland for compressing the rings around the shaft, as described above with reference to the Bissell patent. The Boutros et al patent also describes the difficulty of obtaining a seal with the stuffing box construction because of seepage through the packing along the shaft with an attendant contamination and maintenance problem. The Boutros et al patent discloses a so-called "mechanical seal" for an agitator shaft which affords a substantially fluid tight seal which is constructed so that a damaged seal may be replaced without disassembly or removal of the shaft from the apparatus. The mechanical seal comprises a stationary sealing ring of metal which is supported through intermediate members on the wall of the tank. A second sealing ring of low friction material, such as carbon or brass, is mounted on the shaft for rotation therewith The sealing rings have polished axial faces which are held in sealing engagement with each other by a spring. In the event of breakage or other failure of a seal ring, an auxiliary sealing means is provided to seal the shaft opening while the sealing ring is repaired. The auxiliary sealing means comprises a ring-shaped member nonrotatably mounted on the shaft and carrying an O-ring seal on its periphery. The shaft is axially movable to engage the auxiliary sealing means with a stationary collar which receives the O-ring in sealing engagement. The auxiliary sealing means is locked in place by a small angular rotation of the shaft to engage a pair of locking lugs with a thread element or ramp to maintain the auxiliary sealing means in position during repair.
Other prior art arrangements which provide an agitator shaft shut-off seal similar to that of the Boutros et al patent are disclosed in the Liddiard U.S. Pat. No. 4,419,015 granted Dec. 6, 1983 and the Larkins U.S. Pat. No. 4,878,677 granted Nov. 7, 1989. These patents both describe a stuffing box seal for an agitator shaft together with a shut-off seal which is closed when the shaft is axially displaced.
A fluid mixing unit for portable containers comprising an impeller module affixed to the container and a power module which is demountably secured to the impeller module is disclosed in the LeMaster U.S. Pat. No. 4,813,786 granted Mar. 21, 1989. In this device, the shaft is sealed by a pair of packing rings which are mounted in an end cap which surrounds the shaft and is supported by a sleeve or barrel member extending into the tank. When the drive module is removed from the impeller module, the opening through the mounting flange in the wall of the tank is closed by a threaded sealing plug.
A general object of this invention is to provide an improved arrangement for preventing leakage around an impeller shaft of a mixing tank and to overcome certain disadvantages of the prior art.